Polymer membranes having air permeability through their thickness are used in various applications such as use as air-permeable membranes and use as sound-permeable membranes. Polymer membranes having air permeability but not having water perviousness (water permeability) through their thickness can be used in applications requiring waterproofness, such as use as waterproof air-permeable membranes and/or waterproof sound-permeable membranes. Typical examples of the former type of polymer membranes are non-woven fabrics. Non-woven fabrics are membranes produced by entangling long fibers or short fibers without weaving. Air can pass through the thickness of non-woven fabrics via interstices between randomly entangled fibers.
Other examples of polymer membranes having air permeability through their thickness are stretched porous membranes having a structure in which a huge number of pores resulting from stretching are distributed. Patent Literature 1 discloses a waterproof air-permeable membrane consisting of a stretched porous membrane of polytetrafluoroethylene (PTFE) or ultra-high-molecular-weight polyethylene.